Unipolar brush cells have only a single dendrite that ends as a tight bunch of branchlets that resemble a brush. These cells receive excitatory input from mossy fibers and inhibitory input from Golgi cells. It is thought that they synapse with granule and Golgi cells, which would make these cells an excitatory feedforward link in the mossy fiber–parallel fiber pathway.
Candelabrum cells are GABAergic cells located in the Purkinje layer. Their dendrites and axons terminate in the molecular layer, where the axonal arborization pattern resembles a candelabrum.
The cerebellar nuclei are the main targets of the cerebellar cortex. This projection is topographically organized in such a way that each longitudinal strip of cortex targets a specific region of the cerebellar nuclei. The gross pattern is that the vermis projects to the fastigial and vestibular nuclei, the paravermal region projects to the interpositus, and the lateral hemisphere projects to the dentate nucleus.
The cerebellar nuclear neurons in turn provide the output from the cerebellum to the rest of the brain (with the primary exception of Purkinje cells that project to the
CHAPTER 9 Organization of Motor Function vestibular nuclei). In discussing the output of the cerebellar nuclei, it is useful to group the nuclear cells according to whether they are GABAergic because the GABAergic cells project back to the inferior olivary nucleus and form a negative-feedback loop to one of the cerebellum’s principal afferent sources. Of importance is that GABAergic cells project to the specific part of the inferior olivary nucleus from which they receive input and from which their overlying longitudinal strip of cortex receives climbing fibers. Thus the cerebellar cortex, cerebellar nuclei, and inferior olivary nucleus are functionally organized as a series of closed loops. The non-GABAergic, excitatory nuclear cells project to a variety of targets from the spinal cord to the thalamus. In general, each nucleus gives rise to crossed ascending and descending projections that leave the cerebellum via the superior cerebellar peduncle. The fastigial nucleus also gives rise to significant uncrossed fibers, as well as a second crossed projection called the uncinate gyrus, or hook bundle, that leaves via the inferior cerebellar peduncle.
Although there are differences in the specific targets of each nucleus, in general, the ascending cerebellar projections target midbrain structures, such as the red nucleus and superior colliculus, and the ventral lateral nucleus of the thalamus, which connects to the primary motor cortex and thereby links the cerebellum to motor areas of the cerebrum. (The cerebral motor areas are likewise linked to the cerebellum by multiple pathways, including ones that relay in the basilar pons and inferior olivary nucleus.) It should also be mentioned that ascending cerebellar projections, particularly from the dentate nucleus, also target nonmotor regions of the cerebrum, particularly in the frontal lobe. The descending fibers target mainly the basilar pontine nuclei, inferior olivary nucleus, and several reticular nuclei. Lastly, a small cerebellospinal pathway arises principally from the fastigial nucleus. In addition, the fastigial nucleus has significant projections to the vestibular nuclei.
Activity of Purkinje Cells in the Cerebellar Cortex in the Context of Motor Coordination
Mossy fiber input to the cerebellar cortex, via their excitation of granule cells, causes a Purkinje cell to discharge single action potentials, referred to as simple spikes (